Apparatus and method for reset and stabilization control of a magnetic sensor

ABSTRACT

A magnitude and direction of at least one of a reset current and a second stabilization current (that produces a reset field and a second stabilization field, respectively) is determined that, when applied to an array of magnetic sense elements, minimizes the total required stabilization field and reset field during the operation of the magnetic sensor and the measurement of the external field. Therefore, the low field sensor operates optimally (with the highest sensitivity and the lowest power consumption) around the fixed external field operating point. The fixed external field is created by other components in the sensor device housing (such as speaker magnets) which have a high but static field with respect to the low (earth&#39;s) magnetic field that describes orientation information.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. patent application Ser. No.13/286,026 filed Oct. 31, 2011, which is incorporated herein byreference.

TECHNICAL FIELD

The exemplary embodiments described herein generally relate to the fieldof magnetoelectronic devices and more particularly to CMOS compatiblemagnetoelectronic field sensors used to sense magnetic fields.

BACKGROUND

Sensors are widely used in modern systems to measure or detect physicalparameters, such as position, motion, force, acceleration, temperature,pressure, etc. While a variety of different sensor types exist formeasuring these and other parameters, they all suffer from variouslimitations. For example, inexpensive low field sensors, such as thoseused in an electronic compass and other similar magnetic sensingapplications, generally are Hall effect devices with flux concentratorsor anisotropic magnetoresistance (AMR) based devices. In order to arriveat the required sensitivity and reasonable resistances that mesh wellwith CMOS, the sensing units of AMR sensors are generally on the orderof square millimeters in size, while the auxiliary CMOS associated withhall effect sensors can similarly become large and expensive. For mobileapplications, such AMR sensor configurations are too costly in terms ofexpense, circuit area, and power consumption.

Other types of sensors, such as magnetic tunnel junction (MTJ) sensorsand giant magnetoresistance (GMR) sensors, have been used to providesmaller profile sensors, but such sensors have their own concerns, suchas inadequate sensitivity and being effected by temperature changes. Toaddress these concerns, MTJ, GMR, and AMR sensors have been employed ina Wheatstone bridge structure to increase sensitivity and to eliminatetemperature dependent resistance changes. For minimal sensor size andcost, MTJ or GMR elements are preferred. Typically, a Wheatstone bridgestructure uses magnetic shields to suppress the response of referenceelements within the bridge so that only the sense elements (and hencethe bridge) respond in a predetermined manner. However, the magneticshields are thick and their fabrication requires carefully tuned NiFeseed and plating steps. Another drawback associated with magneticshields arises when the shield retains a remnant field when exposed to astrong (−5 kOe) magnetic field, since this remnant field can impair thelow field measuring capabilities of the bridge structure. To prevent theuse of magnetic shields, a Wheatstone bridge structure may include twoopposite anti-ferromagnetic pinning directions for each sense axis,resulting in four different pinning directions which must beindividually set for each wafer, very often requiring complex andunwieldy magnetization techniques.

A large external field, for example, the field to be measured, that issimilar or greater in magnitude and opposite to the appliedstabilization field can reduce, cancel, or reverse the stabilizationfield, resulting in increase in sensor noise and reset reliabilityproblems, preventing an accurate measurement. Furthermore, a largeexternal field opposite to the magnetic device reset directioneffectively reverses the reset field and therefore the effectiveness ofthe reset operation.

Accordingly, it is desirable to provide a magnetoelectronic sensorfabrication method and layout having a high signal to noise ratio formeasuring various physical parameters. There is also a need for asimple, rugged and reliable sensor that can be efficiently andinexpensively constructed as an integrated circuit structure for use inmobile applications. There is also a need for an improved magnetic fieldsensor and method to overcome the problems in the art, such as outlinedabove. Furthermore, other desirable features and characteristics of theexemplary embodiments will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

A field sensor is configured for resetting sense elements prior tomeasurement of a field.

A first exemplary method for measuring an external field by a pluralityof magnetic sense elements comprises providing a first reset field pulseto orient the magnetic sense elements; providing a first stabilizationfield to the plurality of magnetic sense elements; detecting an externalfield component in the direction of the first stabilization field assensed by the magnetic sense elements; determining a magnitude of atleast one of a second reset field pulse and of a second stabilizationfield; applying at least one of the second reset field pulse prior tomeasuring an external field direction and magnitude, and the secondstabilization field while measuring the external field direction andmagnitude; and measuring the external field direction and magnitude.

A second exemplary method for measuring an external field by a pluralityof magnetic sense elements comprises providing a first reset currentpulse to a first current line adjacent to each of the magnetic senseelements for creating a first reset field pulse; providing a firststabilization current to a first current line adjacent each of themagnetic sense elements for creating a first stabilization field;determining an external field component in the direction of the firststabilization field as sensed by circuitry coupled to the magnetic senseelements; determining at least one of a second reset current pulse and asecond stabilization current for creating a reset field pulse and asecond stabilization field, respectively; applying at least one of thesecond reset current pulse prior to measuring external field directionand magnitude and the second stabilization current while measuring anexternal field direction and magnitude; and measuring the external fielddirection and magnitude.

An exemplary magnetic field sensor includes a bridge circuit including aplurality of magnetic sense elements configured to sense an externalfield, the plurality of magnetic sense elements each having an axis; acurrent line adjacent to each of the plurality of magnetic senseelements; first circuitry configured to apply a first reset currentpulse and subsequently apply a first stabilization current on thecurrent line; and measure the magnitude and direction of the externalfield as sensed by the plurality of magnetic sense elements anddetermine a component of the external field in the direction of theaxis; second circuitry configured to determine a magnitude and directionof at least one of a second reset current pulse and a secondstabilization current for creating a second reset field pulse and asecond stabilization field, respectively, apply at least one of thereset field prior to measuring external field direction and magnitude orthe second stabilization field while measuring external field directionand magnitude; and measure the external field direction and magnitude

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 illustrates two active sense elements having magnetizations thatare angled equally in different directions from a pinned layer that willdeflect in response to an externally applied magnetic field and providean output signal related to the component of the magnetic field that isnot aligned with the pinning direction of the pinned layer;

FIG. 2 illustrates an electronic compass structure which usesdifferential sensors formed from two bridge structures with unshieldedMTJ sensors, along with the circuit output for each bridge structure;

FIG. 3 is a simplified schematic perspective view of a Wheatstone bridgecircuit in which series-connected MTJ sensors are aligned to havedifferent magnetization directions from the magnetization direction ofthe pinned layer;

FIG. 4 is a partial schematic perspective view of first and second MTJsensors which include a magnetic field generator structure for clearingor stabilizing the sense layer prior to or during sense operations;

FIG. 5 is a partial cross-sectional view of an integrated circuit inwhich the first and second MTJ sensors shown in FIG. 4 are formed tohave sense layers with different magnetization directions;

FIG. 6 is an example plot of the magneto-resistance against the appliedfield when no stabilization field is applied to the sensor;

FIG. 7 is an example plot of the magneto-resistance against the appliedfield when a steady state stabilization field is applied to the sensor;

FIG. 8 is an example plot of the magneto-resistance against the appliedfield when a pre-measurement reset pulse is applied to the sensor;

FIG. 9 is a simplified schematic top or plan view of a reticle layoutshowing differential sensor formed with a plurality of series-connectedMTJ sensors configured in a Wheatstone bridge circuit with a magneticfield generator structure positioned in relation to the MTJ sensors;

FIG. 10 is an electronic compass structure which uses differentialsensors formed from three bridge structures with MTJ sensors inaccordance with an exemplary embodiment;

FIG. 11 is a partial cross section of the Z axis bridge structure ofFIG. 10 in accordance with another exemplary embodiment;

FIG. 12 is a view of flux lines as calculated by finite elementsimulation of two of the four magnetic tunnel junction sensors of FIG.11;

FIG. 13 is a partial block diagram of three Wheatstone bridgesconfigured to receive sequential application of reset pulses forresetting a plurality of MTJ sensors within each bridge;

FIG. 14 is a partial block diagram of another three Wheatstone bridgesconfigured to receive sequential resetting pulses for resetting aplurality of MTJ sensors within each bridge and sensing a magneticfield;

FIG. 15 is a flow chart showing a method of sequentially resetting theMTJ field sensors which may be used to provide initialize magnetic fieldsensing elements.

FIG. 16 is a block diagram of the logic circuit for performing themethods in accordance with the exemplary embodiments;

FIG. 17 is a flowchart showing a first exemplary method for reset andstabilization control of a magnetic sensor utilizing a non-orthogonaloffset between sense element easy axes and the reference layermagnetization angle;

FIG. 18 is a flowchart showing a exemplary method for reset andstabilization control of a magnetic sensor with a 90 degree orientationbetween sense element easy axis and the reference layer magnetizationangle; and

FIG. 19 is a flowchart showing a third exemplary method for reset andstabilization control of a magnetic sensor.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

A method and apparatus are described for a differential sensor array inwhich sense elements formed over pinned layers are dynamically reset andstabilized with reset and stabilization field pulses, whose currentmagnitude and direction are determined from a sensed external fieldcomponent, and are applied (e.g., during each measurement cycle) to thedifferential sensor array. Using shape anisotropy, the shapes of twosense element arrays may be formed to have magnetizations that areangled equally in different directions from a single magnetizationdirection of the pinned layer so that the sense layers will deflect inresponse to an externally applied magnetic field. With thisconfiguration, a single axis magnetic sensor may be formed from a singlepinning direction, or a compass circuit may be formed from three suchdifferential sensors circuits so that only two pinning directions arerequired for the entire three axis compass, thereby simplifying andreducing the manufacturing cost and complexity. In an exampleimplementation, each differential sensor circuit is constructed as aWheatstone bridge structure in which four arrays of unshielded activesense elements are used to detect and measure an externally appliedmagnetic field.

To address field fluctuations that can impair the field response of asense element, the sensor layers may be dynamically stabilized byapplying a reset field pulse either before each field measurement or ata predetermined interval to prepare the magnetic sensor, therebyeliminating the need for any hard bias layer(s) to stabilize the senseelements. Without such hard bias stabilization or a measurementpreparation capability (reset field), a momentary exposure to a largefield may reorient the magnetization of the sense elements in a poorlydetermined state. The sense array is optimally sized for highestpossible signal-to-noise (SNR) while allowing the available voltagesupply to stabilize the sensors with the required current and theresulting stabilization field during the measurement phase. For thesmallest possible physical array size, and hence the lowest cost, acopper line (or parallel connected series of line segments) ispreferably routed underneath and above each sense element. In thestabilization (or measurement) phase, each of these line segments isconnected in series so that each sense element has the identicalstabilization field and hence identical sensor response. Therefore, thearray averaging is precise and SNR is increased the maximal amount for agiven array size. To prepare the sense elements for field measurement,an orienting reset field pulse is applied along the stabilization pathprior to applying the stabilization field.

This reset may be periodic, precede each measurement, or only occur whenan error condition (very high bridge offset indicating misorientation,linearity errors, or high noise condition) is encountered. As theindividual sense element anisotropy is large compared with thestabilization field that must be applied for noise optimization but onthe same order as the field required to reorient the sense element, themagnitude of the orientation field pulse is much larger than thatapplied during the measurement phase for sensor stabilization.Therefore, the available voltage is insufficient to reset an array thatis sized for maximal SNR. Subsequent to the preparation phase, all linesegments within a bridge or sense axis are connected in series, andstabilization current is applied to these segments and a measurement mayproceed.

In many compass applications, the magnetometer most operate in thepresence of a static, but large (on the scale of the 0.5 Gauss Earth'smagnetic field) interfering field that indicates sensor orientation. Asthe magnitude and direction of this interfering field are determined bymounting geometry in the final device (i.e. distance from the speakermagnet in a particular cell phone model), and vary from device todevice, it is desirable to utilize an adaptive method stabilization andsensor resetting method. Such a method may change the amplitude andpolarity of the reset pulse and stabilization fields differently foreach sensor axis, to overcome the static field projects, work in tandemwith it to optimize device signal, power and reset reliability for eachparticular device and field profile, depending upon the projection ofthe interfering field on each sense axis. In order to mitigate theimpact of the external field conflicting with the stabilization field, afirst stabilization current is applied to a current line adjacent themagnetic sense elements and an external field component aligned with astabilization field and created by the first stabilization current isdetermined. In a first exemplary embodiment, a magnitude and directionof a second stabilization current (that produces a second stabilizationfield) is determined that, when applied to the magnetic sense elements,nullify this conflict during measure of the external field. In anotherexemplary embodiment, a magnitude and direction of a reset current (thatproduces a reset field) is determined that, when applied to the magneticsense elements, nullify this conflict during measure of the externalfield. In yet another exemplary embodiment, a hard axis field is appliedby applying a current through a self test line during reset. In stillanother exemplary embodiment, the direction of the second stabilizationfield and the reset field is reversed from the first stabilization fieldif the external field is opposite to the internal stabilization field.If the absolute value of the second stabilization field is greater thana threshold, the second stabilization field is not applied as theexternal field direction is coincident with the desired stabilizationfield and can be utilized in lieu of the internal stabilization field.If the absolute value of the second stabilization field is not greaterthan a threshold, the second stabilization field is applied, and theexternal field direction and strength is measured by the magneticsensor.

Various illustrative embodiments of the present invention will now bedescribed in detail with reference to the accompanying figures. Whilevarious details are set forth in the following description, it will beappreciated that the present invention may be practiced without thesespecific details, and that numerous implementation-specific decisionsmay be made to the invention described herein to achieve the devicedesigner's specific goals, such as compliance with process technology ordesign-related constraints, which will vary from one implementation toanother. While such a development effort might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure. Inaddition, selected aspects are depicted with reference to simplifiedcross sectional drawings without including every device feature orgeometry in order to avoid limiting or obscuring the present invention.It is also noted that, throughout this detailed description,conventional techniques and features related to magnetoresistive RandomAccess Memory (MRAM) design, MRAM operation, semiconductor devicefabrication, and other aspects of the integrated circuit devices may notbe described in detail herein. While certain materials will be formedand removed to fabricate the integrated circuit sensors as part of anexisting MRAM fabrication process, the specific procedures for formingor removing such materials are not detailed below since such details arewell known and not considered necessary to teach one skilled in the artof how to make or use the present invention. Furthermore, thecircuit/component layouts and configurations shown in the variousfigures contained herein are intended to represent exemplary embodimentsof the invention. It should be noted that many alternative or additionalcircuit/component layouts may be present in a practical embodiment.

It will be appreciated that additional processing steps will be used tofabricate of MTJ sensor structures. As examples, one or more dielectric,ferromagnetic and/or conductive layers may be deposited, patterned andetched using well known techniques, along with conventional backendprocessing (not depicted), typically including formation of multiplelevels of interconnect that are used to connect the sensor structures ina desired manner to achieve the desired functionality. Thus, thespecific sequence of steps used to complete the fabrication of thesensor structures may vary, depending on the process and/or designrequirements.

A method for measuring an external field by a plurality of magneticsense elements comprises providing a a first reset field pulse followedby a first stabilization field to the plurality of magnetic senseelements; detecting an external field component in the direction of thefirst stabilization field as sensed by the magnetic sense elements;determining a magnitude of at least one of a second reset field pulseand of a second stabilization field needed to optimize the performanceof the magnetic sensor given the strength and orientation of theexternal field; applying at least one of: the second reset field priorto measuring external field direction and magnitude, and the secondstabilization field while measuring the external field direction andmagnitude; and measuring the external field direction and magnitude.

Turning now to FIG. 1, a sensor structure 1 is shown in simplifiedschematic form which uses two active sense element types 20, 30 and apinned layer 10 to measure an external magnetic field. As depicted, themagnetization directions 21, 31 of the active sense elements 20, 30 areangled equally and in different directions from the magnetizationdirection of a pinned layer 10. To this end, the sense elements 20, 30may be formed so that the shape of each sense element is elongated inthe direction of the desired magnetization for that sense element. Thus,sense elements 20, 30 use their shape anisotropy to create magnetizationdirections that are offset from the pinned layer 10. For example, thefirst sense element 20 may be formed so that its preferred magnetizationdirection is angled at −135 degrees from the magnetization direction ofthe pinned layer 10, and with the second sense element 30 so that itspreferred magnetization direction is angled at 135 degrees from themagnetization direction of the pinned layer 10, although other offsetangles may be used.

Because the conductance across a sense element and pinned layer dependson the cosine of the angle between the sense element and the pinnedlayer, the conductance of the sensor structure can be changed byapplying an external magnetic field (H) which deflects the magnetizationof the sensor elements 20, 30. For example, if there is no applied field(H=0) to a sensor structure 1, then the magnetization directions 21, 31of the sense elements 20, 30 are unchanged, and there is no differencebetween the conductance of the first and second sensor elements 20, 30.And if an external field H is applied to a sensor structure 2 that isdirected along or anti-parallel to the pinned layer 10, the appliedfield will deflect or rotate the magnetic moments 22, 32 of the sensorelements 20, 30 equally, resulting in equal conductance changes for eachsense element, and hence no change in their difference. However, when anexternal field H is applied to a sensor structure 3 that is orthogonalto the pinned layer 10, the magnetic moments 23, 33 for each senseelement 20, 30 are changed differently in response to the applied field.For example, when the external field H shown in FIG. 1 is directed tothe right, the conductance of the first sense element 20 is increased,while the conductance of the second sense element 30 is reduced,resulting in a difference signal that is related to the field strength.In this way, the depicted sensor structure measures the projection ofthe applied field perpendicular to the pinned axis, but not parallel toit.

FIG. 2 shows first and second sensors 201, 211 for detecting thecomponent directions of an applied field along a first y-axis (Axis 1)and a second x-axis (Axis 2), respectively. As depicted, each sensor isformed with unshielded sense elements that are connected in a bridgeconfiguration. Thus, the first sensor 201 is formed from the connectionof sense elements 202-205 in a bridge configuration over a pinned layer206 that is magnetized in a first direction. In similar fashion, thesecond sensor 211 is formed from the connection of sense elements212-215 in a bridge configuration over a pinned layer 216 that ismagnetized in a second direction that is perpendicular to themagnetization direction of the pinned layer 206. In the depicted bridgeconfiguration 201, the sense elements 202, 204 are formed to have afirst magnetization direction and the sense elements 203, 205 are formedto have a second magnetization direction, where the first and secondmagnetization directions are orthogonal with respect to one another andare oriented to differ equally from the magnetization direction of thepinned layer 206. As for the second bridge configuration 211, the senseelements 212, 214 have a first magnetization direction that isorthogonal to the second magnetization direction for the sense elements213, 215 so that the first and second magnetization directions areoriented to differ equally from the magnetization direction of thepinned layer 216. In the depicted sensors 201, 211, there is noshielding required for the sense elements, nor are any special referenceelements required. In an example embodiment, this is achieved byreferencing each active sense element (e.g., 202, 204) with anotheractive sense element (e.g., 203, 205) using shape anisotropy techniquesto establish the easy magnetic axes of the referenced sense elements tobe deflected from each other by 90 degrees.

By positioning the first and second sensors 201, 211 to be orthogonallyaligned with the orthogonal sense element orientations in each sensorbeing deflected equally from the sensor's pinning direction, the sensorscan detect the component directions of an applied field along the firstand second axes. This is illustrated in FIG. 2 with the depicted circuitsimulation shown below each sensor. In each simulation, the simulatedbridge output 207, 217 is a function of an applied field angle for senseelements with an anisotropy field of 10 Oe, applied field of 0.5 Oe, anda magnetoresistance of 100% when the sense element switches from ananti-parallel state to a parallel state. The simulated bridge outputscan be used to uniquely identify any orientation of the applied externalfield. For example, a field that is applied with a 0 degree field angle(e.g., pointing “up” so that it is aligned with the y-axis or Axis 1)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of 10 mV/V from the second sensor 201.Conversely, a field that is applied in the opposite direction (e.g.,pointing “down” so that it is aligned with a 180 degree field angle)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of −10 mV/V from the second sensor 201.

As seen from the foregoing, a magnetic field sensor may be formed fromdifferential sensors 201, 211 which use unshielded sense elements202-205, 212-215 connected in a bridge configuration over respectivepinned layers 206, 216 to detect the presence and direction of anapplied magnetic field. With this configuration, the possibility ofresidual magnetic moment present in magnetic shielding, or NiFe fluxconcentrators and flux guides; such as may be present for hall deviceswith triaxial response, is eliminated is eliminated. In addition, themagnetic field sensor provides good sensitivity, and also provides thetemperature compensating properties of a bridge configuration. Byeliminating the need to form magnetic shielding layers, themanufacturing complexity and cost is reduced and the size of the sensorstructure is decreased (in terms of eliminating the silicon real estaterequired to form any shielding layers). There are also performancebenefits to using unshielded sense elements since the magnetic remnanceproblem is eliminated by removing the magnetic shielding layers.

FIG. 3 provides a simplified schematic perspective view of an examplefield sensor 300 formed by connecting four MTJ sensors 301, 311, 321,331 in a Wheatstone bridge circuit, where the series-connected MTJsensors 301, 311, 321, 331 are formed with sense layers 302, 312, 322,332 that are aligned to have different magnetization directions from themagnetization direction of the pinned layers 304, 314, 324, 334. Thedepicted sensor 300 is formed with MTJ sensors 301, 311, 321, 331 thatmay be manufactured as part of an existing MRAM manufacturing processwith only minor adjustments to control the orientation of the magneticfield directions for different layers. In particular, each MTJ sensor301, 311, 321, 331 includes a first pinned electrode 304, 314, 324, 334,an insulating tunneling dielectric layer 303, 313, 323, 333, and asecond sense electrode 302, 312, 322, 332. The pinned and senseelectrodes are desirably magnetic materials, for example, and notintended to be limiting, NiFe, CoFe, Fe, CoFeB and the like, or moregenerally, materials whose magnetization can be collectively aligned.Examples of suitable electrode materials and arrangements are thematerials and structures commonly used for electrodes ofmagnetoresistive random access memory (MRAM) devices, which are wellknown in the art and contain, among other things, ferromagneticmaterials. The pinned and sense electrodes may be formed to havedifferent coercive force or field requirements. The coercive field isbasically the amount of field that is required to reverse the magnetfrom one direction to another after saturation. Technically, it is themagnetic field required to return the magnetization of the ferromagnetto zero after it has been saturated. For example, the pinned electrodes304, 314, 324, 334 may be formed with an anti-ferromagnetic filmexchange coupled to a ferromagnetic film to with a high coercive fieldso that their magnetization orientation can be pinned so as to besubstantially unaffected by movement of an externally applied magneticfield. In contrast, the sense electrodes 302, 312, 322, 332 may beformed with a magnetically soft material to provide different anisotropyaxes having a comparatively low coercive force so that the magnetizationorientation of the sense electrode (in whatever direction it is aligned)may be altered by movement of an externally applied magnetic field. Inselected embodiments, the coercive field for the pinned electrodes isabout two orders of magnitude larger than that of sense electrodes,although different ratios may be used by adjusting the respectivecoercive fields of the electrodes using well known techniques to varytheir composition and/or pinning strength.

As shown in FIG. 3, the pinned electrodes 304, 314, 324, 334 in the MTJsensors are formed to have a first exemplary anisotropy axis alignmentin the plane of the pinned electrode layers 304, 314, 324, 334(identified by the vector arrows pointing toward the top of the drawingof FIG. 3). As described herein, the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 may be obtained using shapeanisotropy of the pinned electrodes, in which case the shapes of thepinned electrodes 304, 314, 324, 334 would each be longer in thedirection of the “up” vector arrow for a single layer pinned magneticstack. In addition or in the alternative, the anisotropy axis alignmentfor the pinned electrodes 304, 314, 324, 334 may be obtained by formingone or more magnetic layers in the presence of a saturating magneticfield that is subsequently or concurrently annealed and then cooled sothat the magnetic field direction of the pinned electrode layers is setin the direction of the saturating magnetic field. As will beappreciated, the formation of the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 must be reconciled with thefabrication steps used to form any other field sensors which includepinned electrodes having a distinct anisotropy axis alignment, as wellas any fabrication steps used to form any sense electrodes having adistinct anisotropy axis alignment.

The depicted field sensor 300 also includes MTJ sensors 301, 321 inwhich sense electrodes 302, 322 are formed to have an exemplaryanisotropy axis (identified by the left-pointing vector arrows) that isoffset from the anisotropy axis of the pinned electrodes by a firstdeflection angle. In addition, the depicted field sensor 300 includesMTJ sensors 311, 331 in which sense electrodes 312, 332 are formed tohave an exemplary anisotropy axis (identified by the right-pointingvector arrows) that is offset from the anisotropy axis of the pinnedelectrodes by a second deflection angle which is equal but opposite tothe first deflection angle. In a particular embodiment, the firstdeflection angle is perpendicular to the second deflection angle so thatanisotropy axis of the sense electrodes 302, 322 is rotated negative 45degrees with respect to the anisotropy axis of the pinned electrodes,and so that anisotropy axis of the sense electrodes 312, 332 is rotatedpositive 45 degrees with respect to the anisotropy axis of the pinnedelectrodes.

As will be appreciated, the MTJ sensors 301, 311, 321, 331 may be formedto have identical structures that are connected as shown in series bymetal interconnections in a standard Wheatstone bridge circuitconfiguration with both power supply terminals 341, 343 and outputsignal terminals 342, 344 for the bridge circuit being shown. Byconnecting in series the unshielded MTJ sensors 301, 311, 321, 331 in aWheatstone bridge circuit, the field sensor 300 detects the horizontaldirection (left to right in FIG. 3) component of an externally appliedmagnetic field, thereby forming an X-axis sensor bridge. In particular,a horizontal field component would deflect the magnetization of thesense electrodes 302, 322 differently from the deflection of themagnetization of the sense electrodes 312, 332, and the resultingdifference in sensor conductance/resistance would quantify the strengthof the horizontal field component. Though not shown, a Y-axis sensorbridge circuit may also be formed with unshielded MTJ sensors connectedin a Wheatstone bridge circuit configuration, though the anisotropy axisof the pinned electrodes in the Y-axis sensor bridge circuit would beperpendicular to the anisotropy axis of the pinned electrodes 304, 314,324, 334 in the X-axis sensor bridge. Each of these sensors (or bridgelegs) 301, 311, 321, 331 may represent an array of sense elementsworking in concert to increase the overall SNR of the system.

Low field magnetic sensors are susceptible to Barkhausen noise, sporadicde-pinning, jumps of micro-magnetic domains resulting from differentregions in the magnetic sense element that may have slightly differentorientations of their local magnetic moment from weak localizedmagnetization pinning caused by edge roughness, or small localinhomogeneities in the sense layer, or a myriad of other sources. Suchnoise can introduce errors in accurately measuring the angularresolution of the Earth's magnetic field. When a field is applied, thesemicro-magnetic domains may reverse in a sequential fashion in lieu ofthe desired coherent rotation of the sense element. Prior attempts toaddress such noise have used a hard magnetic bias layer in the senselayers to pin the ends of the device. However, hard bias layers canreduce the sensitivity of the sensor, and have the additionaldisadvantages of requiring an additional processing layer, etch step andanneal step.

To address the Barkhausen noise problem, a magnetic field may beselectively applied along the easy axis of the sense element prior toperforming a measurement. In selected embodiments, the magnetic field isapplied as a brief field pulse that is sufficient to restore themagnetic state of the sense element and remove micro-magnetic domainsthat may have appeared as the result of exposure to a strong field. Inan example implementation, a field pulse is applied to a sensor toremove metastable pinned regions in the sense element, where the fieldpulse has a threshold field strength (e.g., above approximately 40 Oe)and a minimum pulse duration (e.g., of approximately 2-100 nanoseconds).By applying such a field pulse with a predetermined measurement period(e.g., 10 Hz) as required for a compass application, the resulting fieldpulse has an extremely low duty cycle and minimal power consumption. Inaddition, by terminating the field pulse prior to measurement, there isno additional field applied to the sense element during measurement,resulting in maximal sensitivity. Alternatively, a much lowerstabilizing field may be applied through the same reset line duringsensor measurement, minimally impacting the sensitivity, but encouragingclean coherent rotation of the sense element magnetization.Additionally, the field imposed by magnetic sense elements within thedevice housing the sensor may be taken into account to calculate theoptimal direction and magnitude of the stabilization and reset field.

To illustrate an example of how a field pulse may be applied to a senseelement, reference is now made to FIG. 4, which shows a partialschematic perspective view of first and second MTJ sensors 410, 420which each include a magnetic field generator structure 414, 424 forresetting or stabilizing the sense layer 411, 421 prior to or duringsense operations. Each MTJ sensor may be constructed as shown in FIG. 4where the magnetic direction of the sense layer determines theorientation of the magnetic field generator structure. In particular,each MTJ sensor generally includes an upper ferromagnetic layer 411,421, a lower ferromagnetic layer 413, 423, and a tunnel barrier layer412, 422 between the two ferromagnetic layers. In this example, theupper ferromagnetic layer 411, 421 may be formed to a thickness in therange 10 to 10000 Angstroms, and in selected embodiments in the range 10to 100 Angstroms, and functions as a sense layer or free magnetic layerbecause the direction of its magnetization can be deflected by thepresence of an external applied field, such as the Earth's magneticfield. As for the lower ferromagnetic layer 413, 423, it may be formedto a thickness in the range 10 to 2000 Angstroms, and in selectedembodiments in the range 10 to 100 Angstroms, and functions as a fixedor pinned magnetic layer when the direction of its magnetization ispinned in one direction that does not change magnetic orientationdirection during normal operating conditions. As described above, thefirst and second MTJ sensors 410, 420 may be used to construct adifferential sensor by forming the lower pinned layers 413, 423 to havethe same magnetization direction (not shown), and by forming themagnetization direction 415 in upper sense layer 411 to be orthogonal tothe magnetization direction 425 in upper sense layer 421 so that themagnetization directions 415, 425 are oriented in equal and oppositedirections from the magnetization direction of the lower pinned layers413, 423.

To restore the original magnetization of the upper sense layers 411, 421that can be distorted by magnetic domain structure, FIG. 4 depicts amagnetic field generator structure 414, 424 formed below each sensor. Inselected embodiments, the magnetic field generator structure 414, 424 isformed as a conducting current line which is oriented to create amagnetic field pulse which aligns with the magnetization direction 415,425 in the upper sense layer 411, 421. For example, when a current pulseflows through the magnetic field generator structure 414 below the firstMTJ sensor 410 in the direction indicated by the arrow 416, a fieldpulse is created that is aligned with the easy axis 415 of the senseelement 411 in the first MTJ sensor 410. However, since the second MTJsensor 420 has a sense layer 421 with a different magnetizationdirection 425, the magnetic field generator structure 424 is oriented sothat a field pulse is created that is aligned with the easy axis 425 ofthe sense element 421 in the second MTJ sensor 420 when a current pulseflows through the magnetic field generator structure 424 in thedirection indicated by the arrow 426.

Various illustrative embodiments of the process integration will now bedescribed in detail with reference to the accompanying figures. Whilevarious details are set forth in the following description, it will beappreciated that the present invention may be practiced without thesespecific details, and that numerous implementation-specific decisionsmay be made to the invention described herein to achieve the devicedesigner's specific goals, such as compliance with process technology ordesign-related constraints, which will vary from one implementation toanother. While such a development effort might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure. Inaddition, selected aspects are depicted with reference to simplifiedcross sectional drawings without including every device feature orgeometry in order to avoid limiting or obscuring the present invention.It is also noted that, throughout this detailed description,conventional techniques and features related to magnetic sensor designand operation, Magnetoresistive Random Access Memory (MRAM) design, MRAMoperation, semiconductor device fabrication, and other aspects of theintegrated circuit devices may not be described in detail herein. Whilecertain materials will be formed and removed to fabricate the integratedcircuit sensors as part of an existing MRAM fabrication process, thespecific procedures for forming or removing such materials are notdetailed below since such details are well known and not considerednecessary to teach one skilled in the art of how to make or use thepresent invention. Furthermore, the circuit/component layouts andconfigurations shown in the various figures contained herein areintended to represent exemplary embodiments of the invention. It shouldbe noted that many alternative or additional circuit/component layoutsmay be present in a practical embodiment.

The exemplary embodiments described herein may be fabricated using knownlithographic processes as follows. The fabrication of integratedcircuits, microelectronic devices, micro electro mechanical devices,microfluidic devices, and photonic devices involves the creation ofseveral layers of materials that interact in some fashion. One or moreof these layers may be patterned so various regions of the layer havedifferent electrical or other characteristics, which may beinterconnected within the layer or to other layers to create electricalcomponents and circuits. These regions may be created by selectivelyintroducing or removing various materials. The patterns that define suchregions are often created by lithographic processes. For example, alayer of photoresist material is applied onto a layer overlying a wafersubstrate. A photomask (containing clear and opaque areas) is used toselectively expose this photoresist material by a form of radiation,such as ultraviolet light, electrons, or x-rays. Either the photoresistmaterial exposed to the radiation, or that not exposed to the radiation,is removed by the application of a developer. An etch may then beapplied to the layer not protected by the remaining resist, and when theresist is removed, the layer overlying the substrate is patterned.Alternatively, an additive process could also be used, e.g., building astructure using the photoresist as a template.

The relative alignment of the field pulse and easy axis directions mayalso be seen in FIG. 5, which depicts a partial cross-sectional view ofan integrated circuit device in which the first and second MTJ sensorsshown in FIG. 4 are formed to have sense layers 411, 421 with differentmagnetization directions. In particular, the cross-sectional view on theleft shows the first MTJ sensor 410 as seen from the perspective view 5Ain FIG. 4, while the cross-sectional view on the right shows the secondMTJ sensor 420 as seen from the perspective view 5B in FIG. 4. The firstand second MTJ sensors 410, 420 are each formed over a substrate 430,440 which may have an active circuit 431, 441 embedded therein. On thesubstrate, one or more circuit layers 432, 442 may be formed beforeforming an insulating layer 433, 443 in which a conductive line 414, 424is embedded to form a magnetic field generator structure. As shown inFIG. 5, the conductive line 414 in the first MTJ sensor 410 is formed tocarry current in the direction coming out of plane of the drawing ofFIG. 5, while the conductive line 424 in the second MTJ sensor 420 isformed to carry current moving right-to-left on the drawing. Over theembedded conductive lines, the first and second MTJ cores are formed inan insulating layer 435, 445. In particular, the first MTJ core in thefirst MTJ sensor 410 includes a first conductive line 434 at leastpartially embedded in the insulating layer 435, a lower pinnedferromagnetic layer 413, a tunnel barrier layer 412, an upper senseferromagnetic layer 411 having a magnetization direction 415 that isoriented right-to-left, and a second conductive line 436 over which isformed an additional dielectric layer 437. The first conductive layer434 is connected to a bottom contact layer 438 through a via structure439. In addition, the second MTJ core in the second MTJ sensor 420includes a first conductive line 444 at least partially embedded in theinsulating layer 445, a lower pinned ferromagnetic layer 423, a tunnelbarrier layer 422, an upper sense ferromagnetic layer 421 having amagnetization direction 425 that is oriented into the plane of thedrawing of FIG. 5, and a second conductive line 446 over which is formedan additional dielectric layer 447. To connect the first and second MTJsensors 410, 420, the first conductive layer 444 in the second MTJsensor 420 is connected through a via structure (not shown) to a bottomcontact layer (not shown) in the same level as the embedded conductiveline 424, which in turn is connected through one or more vias andconductive layers to the second conductive line 436 from the first MTJsensor 410. With the depicted configuration, current pulses through theembedded conductive line 414 will create a magnetic field pulse 417which is aligned with the easy axis 415 of the sense element 411, andcurrent pulses through the embedded conductive line 424 will create amagnetic field pulse in the region of the sense element 421 (not shown)which is aligned with the easy axis 425 of the sense element 421.

The lower pinned ferromagnetic and pinning antiferromagnetic layers 413,423 may be a material, for example, iridium manganese, platinummanganese, cobalt iron, cobalt iron boron, nickel iron, ruthenium, andthe like, or any combination thereof. The tunnel barrier layers 412, 422may be an insulating material, for example, aluminum oxide or magnesiumoxide. The upper sense ferromagnetic layers 411, 421 may be aferromagnetic material, for example, nickel iron, cobalt iron, cobaltiron boron, ruthenium, and/or the like. The magnetic field generatorstructures 414, 424 may be aluminum, copper, tantalum, tantalum nitride,titanium, titanium nitride or the like, while conductive lines ingeneral may be, for example, aluminum, copper, tantalum, tantalumnitride, titanium, titanium nitride or the like.

The first and second MTJ sensors 410, 420 may be fabricated together ona monolithic integrated circuit as part of a differential sensor byforming sense layers 411, 421 having orthogonal magnetic orientationsthat each differ equally from the magnetic direction of the pinnedlayers 413, 423. In an example process flow, the first step in thefabrication process is to provide a monolithic integrated circuit chipsubstrate which is covered by a dielectric base layer (not shown). Overthe dielectric base layer, magnetic field generator structures 414, 424are formed as embedded lines of conductive material using knowndeposition, patterning and etching processes so that the magnetic fieldgenerator structures 414, 424 are aligned and positioned below thesensors 410, 420 and embedded in an insulating layer (not shown). Uponthe insulating layer, a stack of sensor layers is sequentially formed bydepositing a first conductive layer (to serve after etching as theconductive line 434), one or more lower ferromagnetic layers (to serveafter etching as the lower pinned ferromagnetic layer 413), one or moredielectric layers (to serve after etching as the tunnel barrier layer412), one or more upper ferromagnetic layers (to serve after etching asthe upper sense ferromagnetic layer 411), and a second conductive layer(to serve after etching as the conductive line 436).

While the various ferromagnetic layers may each be deposited and heatedin the presence of a magnetic field to induce a desired magneticorientation, shape anisotropy techniques may also be used to achieve therequired magnetic orientations for the different ferromagnetic layers.To this end, the sensor layer stack is selectively etched with asequence of patterned etch processes to define the pinned and senselayers in the MTJ sensors 410, 420. In a first etch sequence, the shapesof the different pinning layers 413, 423 are defined from the lowerferromagnetic layer(s) by using patterned photoresist to form a firstpatterned hard mask and then performing a selective etch process (e.g.,reactive ion etching) to remove all unmasked layers down to andincluding the unmasked lower ferromagnetic layer(s). The resultingshapes of the etched lower ferromagnetic layers are oriented so thateach pinned layer has shape anisotropy, resulting in a preferredmagnetic orientation along one of its axes. In addition to being formedas long and narrow shapes, additional shaping of the ends of pinnedlayers may be provided so that each of the pinned layers performs morelike a single magnetic domain during the magnetic setting procedure.Using shape anisotropy, the shaped pinned layers 413, 423 may beannealed to set their respective pinning directions.

At this point in the fabrication process, the upper ferromagneticlayer(s) will have been selectively etched to leave a remnant portionunder the first patterned hard mask so that the upper and lowerferromagnetic layer(s) have the same shape. However, the final shape ofthe sense layers will be smaller than the underlying pinned layers, andto this end, a second etch sequence is used to define the final shapesof the different sense layers 411, 421 from the remnant portions of theupper ferromagnetic layer(s). In the second etch sequence, anotherphotoresist pattern is used to form a patterned hard mask over the partsof the remnant upper ferromagnetic layer(s) layer that will form thesense layers. The pattern is selected to define high aspect ratio shapesfor the sense layers when a selective etch process (e.g., reactive ionetching) is used to remove all unmasked layers down to and including theunmasked upper ferromagnetic layer(s) 411, 421. In selected embodiments,the selective etch process may leave intact the underlying shaped pinnedlayers 413, 423, though in other embodiments, the selective etch processalso etches the unmasked portions of the underlying shaped pinned layers413, 423. The defined high aspect ratio shapes for the sense layers areoriented so that the sense layers 411 are longer in the dimension of thedesired magnetization 415 than they are wide, while the sense layers 421are longer in the dimension of the desired magnetization 425 than theyare wide. In other words, the long axis for each sense layer is drawnalong the desired magnetization direction for a single ferromagneticsense layer. In addition to being formed as long and narrow shapes,additional shaping of the ends of sense layers 411, 421 may be providedso that each of the sense layers performs more like a single magneticdomain. For example, the sense layers may be shaped to have pointed endsthat taper in the corresponding directions of desired easy axis for thesense layers. Once the shaped sense layers are formed, the desired easyaxis magnetic orientations may be induced from their shape anisotropy bybriefly annealing the wafer (e.g., at an anneal temperature ofapproximately 250 degrees C. in the absence of a magnetic field toremove material dispersions. Upon cooling, the magnetizations of thesense layers 411, 421 align with the individual pattern, providingmultiple orientations of sense layers.

By controlling the magnitude, direction, and timing of the current flowthrough the magnetic field generator structures 414, 424 so as to createa field pulse just prior to using the sensors 410, 420 to perform afield measurement, the sense layers 411, 421 are prepared before eachmeasurement in a way that maintains high sensitivity and minimizes powerconsumption. While an improved method which is adaptively adjusted forthe magnitude and direction of the external magnetic field from thedevice housing the magnetic sensor will be described below, the benefitsof selectively applying a magnetic field for low or no external magneticfield to the sense elements are demonstrated in FIGS. 6-8. Starting withFIG. 6, there is provided an example plot of the magneto-resistanceagainst the applied field when no stabilization field is applied to thesensor. Without a stabilization field, the micro-magnetic domain jumpscause the transfer curve 60 to have sporadic, unpredictable jumps inmagneto-resistance (a.k.a., Barkhausen noise) as an applied field isswept. This noise may be prevented by applying a weak stabilizationfield in alignment with the easy axis of the sense element. For example,FIG. 7 provides an example plot of the magneto-resistance against theapplied field when a 15 Oe easy axis stabilization field is applied as asteady state field to the sensor. As shown in the plot of FIG. 7, themicro-magnetic domain jumps have been eliminated. As a consequence, thetransfer curve 70 in this example includes a region of linearcharacteristics 71 up to an applied field of approximately 20 Oe. Inaddition or in the alternative, a field pulse may be applied to furtherimprove the transfer curve, as shown in FIG. 8 which provides an exampleplot 80 of the magneto-resistance against the applied field when apulsed stabilization field is applied to the sensor. In particular, thetransfer curve 80 was obtained by briefly pulsing a sensor element alongits easy axis immediately prior to performing a field measurement at thesensor with a sequence of field sweeps, starting with a first sweep from−5 Oe to 5 Oe, and then performing a second sweep from −10 Oe to 10 Oe,and so on. The resulting transfer curve 80 includes a region of linearcharacteristics 81 up to at least an applied field of approximately 20Oe. In addition, the transfer curve 80 indicates that poor performancemay be created when this sensor is exposed to a hard axis field aboveapproximately 40 Oe. Stated more generally, a strong field applied at anarbitrary direction may put the sense element in a bad state, while afield pulse applied along the sensor easy axis is sufficient to removedomain structure from the sense element.

In a practical deployment, the magnetic field generator structures 414,424 are formed from the same layer that is necessary to interconnect thebridge legs, and hence creates no additional processing steps. Inaddition, each of the magnetic field generator structures 414, 424 maybe constructed from a single conductive element that is positioned topass beneath each MTJ sensor with the appropriate orientation, therebycreating field pulses throughout the chip with a single current pulse.While in a practical deployment, each bridge leg will comprise arrays ofsense elements for highest Signal to Noise Ratio (SNR), a simplifiedexample of a single sense element implementation is illustrated in FIG.9, which provides a schematic top or plan view of a reticle layoutshowing differential sensor 900 formed with a plurality ofseries-connected MTJ sensors 921, 922, 923, 924 configured in aWheatstone bridge circuit with a magnetic field generator structure 920positioned in relation to the MTJ sensors. The depicted differentialsensor includes four pinned layers 901, 902, 903, 904 which each havethe same magnetization direction (e.g., a pinned axis in they-direction), as shown by the large vector arrow on each pinned layer.While the pinned layers 901, 902, 903, 904 may be formed using theirshape anisotropy (as indicated in FIG. 9), they may also be formed usinga traditional field-anneal process.

FIG. 9 also shows that two of the MTJ sensors or sensor arrays, 921, 924in the differential sensor are formed with sense layers 911, 914 havinga magnetization direction that is oriented at 45 degrees from vertical,as shown with the easy axis vector pointing to the right in the senselayers 911, 914. The other two MTJ sensors 902, 903 are formed withsense layers 912, 913 having a magnetization direction that is orientedat negative 45 degrees from vertical, as shown with the easy axis vectorpointing to the left in the sense layers 912, 913. While any desiredtechnique may be used to form the sense layers having differentmagnetization directions, selected embodiments of the present inventionuse shape anisotropy techniques to shape the sense elements 911, 914 tohave a magnetization direction (or easy axis) that is oriented atpredetermined deflection angle from vertical, and to shape the senseelements 912, 913 to have a magnetization direction (or easy axis) thatis oriented negatively at the predetermined deflection angle fromvertical. In this way, the magnetization direction of the sense elements911, 914 and the magnetization direction of the sense elements 912, 913are offset equally in opposite directions from the magnetizationdirection of the pinned layers 901, 902, 903, 904.

The depicted differential sensor 900 also includes a magnetic fieldgenerator structure 920 which is formed beneath the MTJ sensors 921,922, 923, 924 so as to selectively generate a magnetic field tostabilize or restore the magnetic field of the sense layers 911, 912,913, 914. In selected embodiments, the magnetic field generatorstructure 920 is formed as a single conductive line which is arranged tocarry current beneath the sense layers 911, 912, 913, 914 in a directionthat is perpendicular to the easy axis orientation of the sense layersso that the magnetic field created by the current is aligned with theeasy axis. Thus, the conductive line 920 is formed below the fourth MTJsensor 924 to create a magnetic field that is aligned with the easy axisof the sense element 914. In addition, the orientation of the conductiveline 920 below the second and third MTJ sensors 922, 923 creates amagnetic field that is aligned with the easy axis of the sense elements912, 913. Finally, the conductive line 920 is formed below the first MTJsensor 921 to create a magnetic field that is aligned with the easy axisof the sense element 911.

Referring to FIG. 10, a magnetic field sensor 100 is formed with first,second, and third differential sensors 101, 111, 121 for detecting thecomponent directions of an applied field along a first axis 120 (e.g.,the y-axis direction), a second axis 110 (e.g., the x-axis direction),and a third axis 130 (e.g., the z-axis direction), respectively. Thez-axis direction is represented as a dot and cross-hairs as going eitherinto or out of the page on which FIG. 10 is situated. Exemplaryembodiments of the first and second sensors 101, 111 are described indetail in U.S. patent application Ser. No. 12/433,679. As depictedherein, each sensor 101, 111, 121 is formed with unshielded senseelements that are connected in a bridge configuration. Thus, the firstsensor 101 is formed from the connection of a plurality of senseelements 102, 103, 104, 105 in a bridge configuration over acorresponding plurality of pinned layers 106-109, where each of thepinned layers 106, 107, 108, 109 is magnetized in the x-axis direction.In similar fashion, the second sensor 111 is formed from the connectionof a plurality of sense elements 112, 113, 114, 115 in a bridgeconfiguration over a corresponding plurality of pinned layers 116, 117,118, 119 that are each magnetized in the y-axis direction that isperpendicular to the magnetization direction of the pinned layers 106,107, 108, 109. Furthermore, the third sensor 121 in the same plane asthe first and second sensors 101, 111 is formed from the connection of aplurality of sense elements 122, 123, 124, 125 in a bridge configurationover a corresponding plurality of pinned layers 126, 127, 128, 129 thatare each magnetized in the xy-axis direction that may be optimallyorientated for the magnetic setting procedure at 45 degrees to themagnetization direction of the pinned layers 106, 107, 108, 109 and 116,117, 118, 119. In other embodiments the third (121) sensor referencelayer direction may be oriented along either x or y axis for optimal(smallest) layout of the finished three axis sensor product die. In thedepicted bridge configuration 101, the sense elements 102, 104 areformed to have a first easy axis magnetization direction and the senseelements 103, 105 are formed to have a second easy axis magnetizationdirection, where the first and second easy axis magnetization directionsare orthogonal with respect to one another and are oriented to differequally from the magnetization direction of the pinned layers 106, 107,108, 109. As for the second bridge configuration 111, the sense elements112, 114 have a first easy axis magnetization direction that isorthogonal to the second easy axis magnetization direction for the senseelements 113, 115 so that the first and second easy axis magnetizationdirections are oriented to differ equally from the magnetizationdirection of the pinned layers 116, 117, 118, 119. In the third bridgeconfiguration 121, the sense elements 122, 123, 124, and 125 all have aneasy axis magnetization direction that is orthogonal to the pinnedmagnetization direction of the pinned layers 126, 127, 128, and 129. Thethird bridge configuration 121 further includes flux guides 132, 133,134, 135 positioned adjacent to the right edge of sense elements 122,123, 124, 125, and flux guides 136, 137, 138, 139 positioned adjacent tothe left edge of sense elements 122, 123, 124, 125, respectively. Fluxguides 132, 137, 134, and 139 are positioned above sense elements 122,123, 124, 125, and flux guides 136, 133, 138, and 135 are positionedbelow sense elements 122, 123, 124, 125. The positioning of these fluxguides 132, 133, 134, 135, 136, 137, 138, 139 is subsequently describedin more detail in FIG. 11. In selected embodiments, the flux guides mayonly utilize one plane; that is only guides 136, 133, 138, and 135, orguides 132, 137, 134, and 139, may be present. In the depicted sensors101, 111, 121 there is no shielding required for the sense elements, norare any special reference elements required. In an exemplary embodiment,this is achieved by referencing each active sense element (e.g., 102,104) with another active sense element (e.g., 103, 105) using shapeanisotropy techniques to establish the easy magnetic axes of thereferenced sense elements to be deflected from each other by 90 degreesfor the x and y sensors, and referencing a sense element that respondsin an opposite manner to an applied field in the Z direction for the Zsensor. The Z sensor referencing will be described in more detail below.The configuration shown in FIG. 10 is not required to harvest thebenefits of the third sensor 121 structure described in more detail inFIG. 11, and is only given as an example.

Still referring to FIG. 10, by positioning the first and second sensors101, 111 to be orthogonally aligned, each with the sense elementorientations deflected equally from the sensor's pinning direction andorthogonal to one another in each sensor, the sensors can detect thecomponent directions of an applied field along the first and secondaxes. Flux guides 132-139 are positioned in sensor 121 above and belowthe opposite edges of the elements 122-125, in an asymmetrical mannerbetween legs 141, 143 and legs 142, 144. As flux guides 132, 134 areplaced above the sense elements 122, 124, the magnetic flux from the Zfield may be guided by the flux guides 132 and 134 into the xy planealong the right side and cause the magnetization of sense elements 122and 124 to rotate in a first direction towards a higher resistance.Similarly, the magnetic flux from the Z field may be guided by the fluxguides 133 and 135 into the xy plane along the right side of the senseelement and cause the magnetization of sense elements 123 and 125 torotate in a second direction, opposite from the first direction towardsa lower resistance, as these flux guides are located below the senseelements 123, 125. Thus, the sensor 121 can detect the componentdirections of an applied field along the third axis. Although in thepreferred embodiment, the flux guides are in a plane orthogonal to theplane of the field sensor, the flux guides will still function if theangle they make with the sensor is not exactly 90 degrees. In otherembodiments, the angle between the flux guide and the field sensor couldbe in a range from 45 degrees to 135 degrees, with the exact anglechosen depending on other factors such as on the ease of fabrication.

As seen from the foregoing, a magnetic field sensor may be formed fromdifferential sensors 101, 111, 121 which use unshielded sense elements102-105, 112-115, and sense elements 122-125 with guided magnetic fluxconnected in a bridge configuration over respective pinned, orreference, layers 106-109, 116-119, and 126-129 to detect the presenceand direction of an applied magnetic field. With this configuration, themagnetic field sensor provides good sensitivity, and also provides thetemperature compensating properties of a bridge configuration.

The bridge circuits 101, 111, 121 may be manufactured as part of anexisting MRAM or thin-film sensor manufacturing process with only minoradjustments to control the magnetic orientation of the various sensorlayers and cross section of the flux guiding structures. Each of thepinned layers 106-109, 116-119, and 126-129 may be formed with one ormore lower ferromagnetic layers, and each of the sense elements 102-105,112-125, 122-125 may be formed with one or more upper ferromagneticlayers. An insulating tunneling dielectric layer (not shown) may bedisposed between the sense elements 102-105, 112-125, 122-125 and thepinned layers 106-109, 116-119, and 126-129. The pinned and senseelectrodes are desirably magnetic materials whose magnetizationdirection can be aligned. Suitable electrode materials and arrangementsof the materials into structures commonly used for electrodes ofmagnetoresistive random access memory (MRAM) devices and other magnetictunnel junction (MTJ) sensor devices are well known in the art. Forexample, pinned layers 106-109, 116-119, and 126-129 may be formed withone or more layers of ferromagnetic and antiferromagnetic materials to acombined thickness in the range 10 to 1000 Å, and in selectedembodiments in the range 250 to 350 Å. In an exemplary implementation,each of the pinned layers 106-109, 116-119, and 126-129 is formed with asingle ferromagnetic layer and an underlying anti-ferromagnetic pinninglayer. In another exemplary implementation, each pinned layer 106-109,116-119, and 126-129 includes a synthetic anti-ferromagnetic stackcomponent (e.g., a stack of CF (Cobalt Iron), Ruthenium (Ru) and CFB)which is 20 to 80 Å thick, and an underlying anti-ferromagnetic pinninglayer that is approximately 200 Å thick. The lower anti-ferromagneticpinning materials may be re-settable materials, such as IrMn, thoughother materials, such as PtMn, can be used which are not readily re-setat reasonable temperatures. As formed, the pinned layers 106-109,116-119, and 126-129 function as a fixed or pinned magnetic layer whenthe direction of its magnetization is pinned in one direction that doesnot change during normal operating conditions. As disclosed herein, theheating qualities of the materials used to pin the pinned layers106-109, 116-119, and 126-129 can change the fabrication sequence usedto form these layers.

One of each of the sense elements 102-105, 112-125, 122-125 and one ofeach of the pinned layers 106-109, 116-119, 126-129 form a magnetictunnel junction (MTJ) sensor. For example, for bridge circuit 121, senseelement 122 and pinned layer 126 form an MTJ sensor 141. Likewise, senseelement 123 and pinned layer 127 form an MTJ sensor 142, sense element124 and pinned layer 128 form an MTJ sensor 143, and sense element 125and pinned layer 129 form an MTJ sensor 144.

The pinned layers 106-109, 116-119, and 126-129 may be formed with asingle patterned ferromagnetic layer having a magnetization direction(indicated by the arrow) that aligns along the long-axis of thepatterned reference layer(s). However, in other embodiments, the pinnedreference layer may be implemented with a synthetic anti-ferromagnetic(SAF) layer which is used to align the magnetization of the pinnedreference layer along the short axis of the patterned referencelayer(s). As will be appreciated, the SAF layer may be implemented incombination with an underlying anti-ferromagnetic pinning layer, thoughwith SAF structures with appropriate geometry and materials that providesufficiently strong magnetization, the underlying anti-ferromagneticpinning layer may not be required, thereby providing a simplerfabrication process with cost savings.

The sense elements 102-105, 112-125, 122-125 may be formed with one ormore layers of ferromagnetic materials to a thickness in the range 10 to5000 Å, and in selected embodiments in the range 10 to 60 Å. The upperferromagnetic materials may be magnetically soft materials, such asNiFe, CoFe, Fe, CFB and the like. In each MTJ sensor, the sense elements102-105, 112-125, 122-125 function as a sense layer or free magneticlayer because the direction of their magnetization can be deflected bythe presence of an external applied field, such as the Earth's magneticfield. As finally formed, sense elements 102-105, 112-125, 122-125 maybe formed with a single ferromagnetic layer having a magnetizationdirection (indicated with the arrows) that aligns along the long-axis ofthe patterned shapes.

The pinned layers 106-109, 116-119, 126-129 and sense elements 102-105,112-125, 122-125 may be formed to have different magnetic properties.For example, the pinned layers 106-109, 116-119, 126-129 may be formedwith an anti-ferromagnetic film exchange layer coupled to aferromagnetic film to form layers with a high coercive force and offsethysteresis curves so that their magnetization direction will be pinnedin one direction, and hence substantially unaffected by an externallyapplied magnetic field. In contrast, the sense elements 102-105,112-125, 122-125 may be formed with a magnetically soft material toprovide different magnetization directions having a comparatively lowanisotropy and coercive force so that the magnetization direction of thesense electrode may be altered by an externally applied magnetic field.In selected embodiments, the strength of the pinning field is about twoorders of magnitude larger than the anisotropy field of the senseelectrodes, although different ratios may be used by adjusting therespective magnetic properties of the electrodes using well knowntechniques to vary their composition.

The pinned layers 106-109, 116-119, 126-129 in the MTJ sensors areformed to have a shape determined magnetization direction in the planeof the pinned layers 106-109, 116-119, 126-129 (identified by the vectorarrows for each sensor bridge labeled “Pinning direction” in FIG. 1). Asdescribed herein, the magnetization direction for the pinned layers106-109, 116-119, 126-129 may be obtained using shape anisotropy of thepinned electrodes, in which case the shapes of the pinned layers106-109, 116-119, 126-129 may each be longer in the pinning directionfor a single pinned layer. Alternatively, for a pinned SAF structure,the reference and pinned layers may be shorter along the pinningdirection. In particular, the magnetization direction for the pinnedlayers 106-109, 116-119, 126-129 may be obtained by first heating theshaped pinned layers 106-109, 116-119, 126-129 in the presence of aorienting magnetic field which is oriented non-orthogonally to the axisof longest orientation for the shaped pinned layers 106-109, 116-119,126-129 such that the applied orienting field includes a field componentin the direction of the desired pinning direction for the pinned layers106-109, 116-119, 126-129. The magnetization directions of the pinnedlayers are aligned, at least temporarily, in a predetermined direction.However, by appropriately heating the pinned layers during thistreatment, and removing the orienting field, and then further heatingabove the phase transition temperature of the pinning anti-ferromagneticmaterial, the magnetization of the pinned layers relaxes along thedesired axis of orientation for the shaped pinned pinned layers 106-109,116-119, 126-129. Once the magnetization relaxes, the pinned layers canbe annealed and/or cooled so that the magnetic field direction of thepinned electrode layers is set in the desired direction for the shapedpinned layers 106-109, 116-119, 126-129.

The MTJ sensors (141-144) and the flux guide placement within sensor 121(FIG. 10) are now further described. Referring to FIG. 11, the structureof the MTJ devices 141-144 of the third bridge circuit 121 includes thecladded lines 145-149 divided into two independent metal lines, andadditional non-flux guiding cladding (161-168 and 191-198) is placed inbetween these two metal lines at the interior edges, all formed withinthe dielectric material 140. For sensor 141, the flux guide 161 on theleft edge of the left metal line, 148 guides Z field flux into the senseelement 122 to its left, and the flux guide 192 on the right most edgeof the right metal line 145 guides Z field flux into the sense element122 on its right. Sensors 142-144 function similarly, with the claddededge of the metal line adjacent to each sense element serving the activeflux guiding function. As these lines are separated, a current may bemade to pass through cladded lines 145, 146, 182 and 183 into the page,and 181, 147, 148, and 149 out of the page to create a magnetic fieldalong the cladded line edges with a Z component pointing in a consistentdirection (down in this example). These current orientations can serveto create a magnetic field with a strong component in the Z direction,which, through a calibration for the geometry can serve as a self testfor the functionality and sensitivity of the Z axis response. The lines145-149, 181-183 are preferably copper, but in some embodiments may be adielectric. A metal stabilization line 150 is positioned above the MTJdevices 141-144 for providing a stabilization field to the senseelements. The ends of the flux guides may be brought as close aspossible to the sensor elements, with a preferable spacing of less thanor equal to 250 nm between the two. The sense elements are brought asclose as possible for the tightest density array, preferably less than2.5 um apart.

FIG. 12 is a view of flux lines as calculated by finite elementsimulation of MTJ devices 141, 142 of FIG. 11 with a magnetic field inthe z direction imparted upon the sense elements 122-123. FEM modelingshows the resultant magnetic flux lines 160, exhibiting a component inthe plane of the sensor. MTJ device 141 is represented by flux guides132 and 136 on opposed ends of the sensing element 122. MTJ device 142is represented by flux guides 133 and 137 on opposed ends of the sensingelement 123. Stated otherwise, sensing element 122 extends from fluxguides 132 and 136, and sensing element 123 extends from flux guides 133and 137. The magnetic field 160 in the Z-axis 130 produces an asymmetricresponse in the sensing elements 122, 123 along the X-axis 120 asindicated by the arrows 170. In this manner, for a field 160 in the Zdirection 130 directed towards the bottom of the page, the magnetizationof sense element 122 rotates away from the pinning direction (and tohigher resistance) of the pinned layer 126, while the magnetization ofsense element 123 rotates towards the pinning direction (and to lowerresistance) of pinned layer 127. For a field in the X direction 120,both elements 122, 123 show induced magnetization in the same direction(towards higher or lower resistance). Therefore, by wiring MTJ elements141, 142 in a Wheatstone bridge for differential measurement andsubtracting the resistances of MTJ devices 141, 142, the X fieldresponse is eliminated and twice the Z field response is measured.

Referring again to FIG. 11, self test lines 145-149 are implemented asconductive lines extending into the page (perpendicular to thestabilization line 150). For example, self test line 145 (Cu filled fluxguide trench) is positioned below sense element 122. A hard axis fieldis created by applying a current through the self test lines 145-149during reset (while a reset current is applied to the stabilization line150). As is well known in the art, this field, when applied togetherwith the reset/stabilization field by 150, lowers the threshold for theeasy axis switching per Stoner Wolforth astroid, and therefore, caneffectively increase the effect of the stabilization or reset field,i.e. reduce the threshold field from the stabilization line 150 withwhich the sensing element may be reset. The self test line may be routedin other layers; e.g., above or below the flux guide or tunnel junction,and hence this hard axis assist field may come from current throughlines other than directly through the flux guide.

Referring to FIG. 13, a block diagram of an integrated magnetic sensor1302 includes the Wheatstone bridge 1304 for sensing a magnetic field inan X direction, a Wheatstone bridge 1306 for sensing a magnetic field ina Y direction, and a Wheatstone bridge 1308 for sensing a magnetic fieldin a Z direction. While three Wheatstone bridges 1304, 1306, 1308 areshown for sensing a field in three dimensions, it is understood thatonly one Wheatstone bridge may be formed for sensing a field in onedimension, or two Wheatstone bridges may be formed for sensing a fieldin two dimensions. One example of a structure for sensing a magneticfield in three dimensions may be found in U.S. patent application Ser.No. 12/567,496. The Wheatstone bridge 1304 includes a first plurality ofmagnetic sense elements 1314, including a first group 1324 and a secondgroup 1334. Likewise, Wheatstone bridge 1306 includes a first pluralityof magnetic sense elements 1316, including a first group 1326 and asecond group 1336, and Wheatstone bridge 1308 includes a first pluralityof magnetic sense elements 1318, including a first group 1328 and asecond group 1338. While two groups are shown in each of the Wheatstonebridges 1304, 1306, 1308, any number of groups numbering two or more maybe utilized. For example, the plurality of magnetic sense elements 1314may comprise i magnetic sense elements, the plurality of magnetic senseelements 1316 may comprise j magnetic sense elements, and the pluralityof magnetic sense elements 1318 may comprise k magnetic sense elements.Each of the magnetic sense elements 1320 within each of the plurality ofmagnetic sense elements 1314, 1316, 1318 comprises a magnetic senseelement such as MTJ sensors 410, 420 of FIGS. 4 and 5. A write driver1325 is configured to provide a current pulse to each of the MTJelements within the group 1324 and a write driver 1335 is configured tosubsequently provide a current pulse to each of the MTJ elements withinthe group 1334. Likewise, a write driver 1327 is configured to provide acurrent pulse to each of the MTJ elements within the group 1326 and awrite driver 1337 is configured to provide a current pulse to each ofthe MTJ elements within the group 1336, and a write driver 1329 isconfigured to provide a sequential current pulse to each of the MTJelements within the group 1328 and a write driver 1339 is configured toprovide a sequential current pulse to each of the MTJ elements withinthe group 1338.

Logic circuit 1341 sequentially selects write drivers 1325, 1335, 1327,1337, 1329, 1339, for sequentially providing a reset current pulse tothe groups 1324, 1334, 1326, 1336, 1328, 1338, respectively. Since agiven voltage is provided, by dividing a reset current line having alarge resistance into several reset current line segments each having asmaller resistance, a larger current may be applied to each group of MTJelements in the groups 1324, 1334, 1326, 1336, 1328, 1338. Also, thereset pulses will be sequential, and a smaller stabilization currentwill be applied during the measurement. This stabilization current willflow through the entire line and stabilize all sense elements with anidentical stabilization field since the voltage is high enough to supplythe required stabilization line over the full line resistance. Thisembodiment is preferred as power consumption is lowest and smalldifferences in stabilization line resistances will not result indifferent stabilization fields. However, for higher SNR ratios than areattainable given this stabilization configuration, stabilization currentmay be applied to different sub groupings of the array in parallel,allowing the possibility of additional stabilization paths (and hencesense element array size increases) within a given sensor.

The logic circuit 1341 is further described in FIG. 14 and includes astate decoder 1402, a reset circuit 1404, and a read bridge selectcircuit 1406. Input signals 1411 and 1412 dictate one of four outputsignals 1413, 1414, 1415, 1416. Output signal 1413 activates the resetcircuit 1404 for sequentially activating each of the drivers 1325, 1335,1327, 1337, 1329, 1339. Output signals 1114, 1115, 1116 activate a readsequence of each of the Wheatstone bridges 1304, 1306, 1308.

Operation of the magnetic sensors described herein may also beillustrated with reference to FIG. 15, which depicts an exemplaryflowchart for a method of operation of magnetic field sensors which donot exhibit micro-magnetic structure by resetting the magnetic senseelements prior to or just after sensing of an external magnetic field.While two bridge circuits are described, it is understood that anynumber of bridge circuits may be utilized. This resetting, orinitializing, includes the steps of applying 1502 a first current pulseto a first reset line positioned adjacent each of a first group ofmagnetic sense elements within a first bridge circuit, applying 1504 asecond current pulse to a second reset line positioned adjacent each ofa second group of magnetic sense elements within the first bridgecircuit, applying 1506 a third current pulse to a third reset linepositioned adjacent each of a third group of magnetic sense elementswithin a second bridge circuit, and applying 1508 a fourth current pulseto a fourth reset line positioned adjacent each of a fourth group ofmagnetic sense elements within the second bridge circuit. Astabilization current is applied 1510 to the stabilization line. Themagnetic sense elements are then accessed 1512 to measure a value of anexternal field projected along each of the sensor axes of the magneticsense elements.

In accordance with the methods of implementing the exemplary embodimentsdescribed herein, FIG. 16 is a block diagram including sense circuitry1702 coupled to a plurality of magnetic sense elements 1604 for sensingan external magnetic field. Logic circuitry 1606 is coupled to the sensecircuitry 1602 for determining a component of the external magneticfield, the component being aligned with the field that is created when acurrent is passed through the reset/stabilization current line 1610. Inresponse to the logic circuit 1606, the current supply circuitry 1608supplies the reset and stabilization currents to the current line 1610adjacent the magnetic sense elements 1604.

As the reset and stabilization fields act primarily along the magneticsense element easy (long) axis in order to properly orient and stabilizethe sense element for measurement, but the sense element magnetizationdeflects from the easy axis in response to a field along the senseelement hard (short) axis, a single axis sensor cannot independentlydetermine the field component that needs to be taken into account inachieve the proper net stabilization and reset field. Accordingly, adual axis sensor is utilized wherein the orthogonal axis determines thecomponent of the external field that lies along the given sense axis'hard axis. For example the field measured by the Y axis sensor isutilized to measure the external field component and to determine theoptimal stabilization and reset field that may be supplied to the X axissensor in its operation. Similarly, the X axis sensor signal is utilizedto determine the stabilization and reset field required to optimallyoperate the Y axis sensor during measurement. In the case of anon-orthogonal offset between the sense element easy axis and thereference layer magnetization angle, a combination of the measurementsfrom each X and Y axis may be utilized to determine the optimalstabilization and reset fields as the projection of the X or Y field onthe sense elements of the Y or X sensor, respectively is non zero asthere are different non-orthogonal sense element orientations within agiven sensor bridge for detection along a single axis. In this case, thereset and stabilization currents may be different values for differentorientation elements within a given axis. For sensors responding to outof plane fields (Z axis sensors) the sense elements themselves arestabilized with in plane fields and therefore it is the in planecomponent of an external field along the sense element easy axis whichmust be optimized for sensor operation. Hence, the signal from the Xsensor, Y sensor, or some combination thereof is utilized to determinethe optimal stabilization and reset field for Z axis operation duringmeasurement.

Referring to FIG. 17, a first exemplary method for reset andstabilization control of a magnetic sensor utilizing a non-orthogonaloffset between sense element easy axis and reference layer magnetizationangle includes applying 1702 a default reset field pulse to a firstplurality of magnetic sense elements in a first bridge, applying 1704 adefault stabilization field to the first plurality of magnetic senseelements of the first bridge, detecting 1706 an external field componentalong the magnetization direction of a pinned layer in the second bridgeas sensed by the magnetic sense elements of the first bridge. Themeasured values are used to change the default stabilization current andreset current pulse that operate the second bridge which detects fieldcomponents along a different sense axis. If the external field componentis aligned with the magnetization direction of the pinned layer in thesecond bridge and greater than a threshold (approaching the senseelement anisotropy), the default reset field pulse direction is reversed1708 and the stabilization field is applied at the original magnitude.The reset field pulse is issued and a measurement of the external fieldis taken 1710 with the default stabilization field on. Reversing thesense element orientation from its original design direction ofapproximately 135 degrees from the pinned layer magnetization reducesthe sensitivity, but also reduces cross axis effects and allows a lowpower measurement. If the measured external field component is alignedwith the magnetization of the pinned layer in the second bridge, greaterthan the default stabilization field but less than about half the senseelement anisotropy, both the default reset field pulse and the defaultstabilization field for the second bridge are increased 1712, but keptin the same polarity wherein they are opposite to the pinned layermagnetization. A measurement with the second bridge is now taken 1710with the newly determined reset field pulse and stabilization fieldamplitudes. This suppresses cross axis effects, keeps the sensor in ahigher sensitivity state, but uses more power to perform themeasurement. If the external field component is aligned with themagnetization of the pinned layer in the second bridge and lower thanabout half the default stabilization field, or opposite to themagnetization of the pinned layer in the second bridge but less thanabout 75% of the default stabilization field, the default stabilizationvalues are used 1714 for a measurement with the second bridge. If themeasured external field component is opposite to the magnetization ofthe pinned layer in the second bridge, but between about 75% of thedefault stabilization field and about 175% of the default stabilizationfield, the default reset field pulse polarity is kept in the defaultdirection, and the stabilization field may be turned off 1716 duringmeasurement with the second bridge. The external field is then used asthe stabilization field. This allows for a higher sensitivity and lowerpower measurement. If the measured external field component is oppositeto the magnetization of the pinned layer in the second bridge, butgreater than about 175% of the default stabilization field, then thedefault stabilization field polarity is reversed 1718, partiallycancelling out the external field component and operating the sensor ata maximal sensitivity. For a multi-axis system, the operating conditionsof each sense axis may be independently determined, depending upon theprojection of the external field upon the initial stabilization fieldsof each axis. This determination of the optimal sensor reset andstabilization configuration need only take place infrequently—perhapseven as infrequently as only once after the sensor has been integratedinto the final system (Cell Phone, GPS, Camera, etc.).

A second exemplary method (FIG. 18), which applies to sensors with a 90degree orientation between sense element easy axis and reference layeraxis includes applying 1802 a first reset field pulse to orient thesense elements in a first bridge which measures the field along a firstaxis, applying 1804 a first stabilization field to a plurality ofmagnetic sense elements in a first bridge, and determining 1806 theprojection of an external field along the magnetization direction (easyaxis) of the sense elements in a second bridge, as sensed by the senseelements of the first bridge. The measured values are used to change thedefault second stabilization current and reset current pulse thatoperate a second bridge which detects field components along a differentsense axis. If the projection of the external field component along themagnetization direction of the sense elements is negative, the resetfield pulse polarity is set to reverse 1808 the magnetization direction.If the magnitude of the external field component is greater than about175% of the default stabilization field, the stabilization field for thesecond sensor is sourced 1812 in the opposite direction from theexternal field component to reduce the net field acting on the senseelements, thereby increasing the sensor sensitivity. If the magnitude ofthe external field component is between about 75% and 175% of thedefault stabilization field, the stabilization field is turned off 1814during measurement, allowing the external field to serve as thestabilization field, and thereby lowering sensor power consumption andincreasing sensitivity over what it would have been had thestabilization field been kept constant. If the magnitude of the externalfield component is lower than about 75% of the default stabilizationfield, the stabilization field is applied 1816 in the same direction asthe external field to ensure that sufficient stabilization field ispresent of linear sensor operation. Once the reset field pulse andstabilization field polarities are determined, the sense elements aresubsequently reset with the newly determined reset field pulse, and thepreviously determined stabilization field values are applied so as totake the sensor measurement under optimal conditions. For a multi-axissystem, the operating conditions of each sense axis may be independentlydetermined, depending upon the projection of the external field upon theinitial stabilization fields of each axis. This determination of theoptimal sensor reset and stabilization configuration need only takeplace infrequently—perhaps even as infrequently as only once after thesensor has been integrated into the final system (Cell Phone, GPS,Camera, etc.).

Alternatively a continuous adjustment of the stabilization current andreset pulse height may be utilized. In this case, a predeterminedoptimal net stabilization field—the combination of the external fieldand the local stabilization field which may be provided from the on chipstabilization current lines—may be targeted for optimal sensor response.The stabilization current applied during operation may be adjustedcontinuously throughout a range for each axis such that the netstabilization field at the sense element array is the desiredpredetermined value. Similarly, the reset field pulse may becontinuously adjusted so that the net reset field achieves apredetermined value.

Referring to FIG. 19, a third exemplary method includes applying 1902 astabilization field to a plurality of magnetic sense elements of a firstbridge, and detecting 1904 an external field component in the easy axisdirection of the magnetic sensing elements of the second bridge. If theexternal field component is higher than a threshold, the switchingthreshold for the magnetic sense elements of the second bridge islowered via resetting 1906 the magnetic sense elements of the secondbridge by applying a reset field pulse while simultaneously applying ahard axis field originated from a current carrying line that is routedat an angle (preferably orthogonal) to the reset line orientation of thesensing elements of the second bridge.

In yet another exemplary embodiment, a method for measuring an ambientmagnetic field with a multi-axis magnetic field sensor having aplurality of magnetic sense elements comprises performing an initialmeasurement using default magnitudes of a first plurality of reset fieldpulses and a first plurality of stabilization fields applied to themagnetic sense elements; determining adjusted magnitudes of a secondplurality of reset field pulses and a second plurality of stabilizationfields based on the initial measurement; performing a second measurementwhile applying the adjusted magnitudes for the second plurality of resetfield pulses and the second plurality of stabilization fields applied tothe magnetic sense elements; and providing output signals representingthe response of the second measurement.

In the abovementioned exemplary methods, the application of the firstreset field and first stabilization field to the first bridge aredescribed to orient and stabilize the sense elements in the firstbridge. They are to provide a more accurate determination of theexternal field component along the first axis. However, these steps canbe optional.

By now it should be appreciated that there has been provided a magneticfield sensor apparatus and a method of operating a plurality ofdifferential sensor circuits over a substrate which detects an appliedmagnetic field directed along one or more axis. The differential sensorcircuits may be constructed as Wheatstone bridge structures, one foreach axis to be sensed, of unshielded magnetic tunnel junction (MTJ)sensors formed with a plurality of pinned layers that are eachmagnetized in a single pinned direction and a corresponding plurality ofunshielded sense layers. In order to nullify the impact of an externalfield conflicting with the reset and stabilization field, a firststabilization current is applied to a current line adjacent the magneticsense elements and an external field component aligned with astabilization field created by the first stabilization current ismeasured. A magnitude and direction of at least one of a reset currentpulse and a second stabilization current (that produces a reset fieldpulse and a second stabilization field, respectively) is determinedthat, when applied to the magnetic sense elements, minimizes this totalrequired stabilization field and reset field pulse during the operationof the magnetic sensor and the measurement of the external field.Therefore the low field sensor operates optimally (with the highestsensitivity and the lowest power consumption) around the fixed externalfield operating point. The fixed external field is created by othercomponents in the sensor device housing (such as speaker magnets) whichhave a high but static field with respect to the low (earth's) magneticfield that describes orientation information.

Although the described exemplary embodiments disclosed herein aredirected to various sensor structures and methods for making same, thepresent invention is not necessarily limited to the example embodimentswhich illustrate inventive aspects of the present invention that areapplicable to a wide variety of semiconductor processes and/or devices.Thus, the particular embodiments disclosed above are illustrative onlyand should not be taken as limitations upon the present invention, asthe invention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. For example, the relative positions of the sense andpinning layers in a sensor structure may be reversed so that the pinninglayer is on top and the sense layer is below. Also the sense layers andthe pinning layers may be formed with different materials than thosedisclosed. Moreover, the thickness of the described layers may vary.Accordingly, the foregoing description is not intended to limit theinvention to the particular form set forth, but on the contrary, isintended to cover such alternatives, modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims so that those skilled in the art shouldunderstand that they can make various changes, substitutions andalterations without departing from the spirit and scope of the inventionin its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

What is claimed is:
 1. A method for operating a plurality of magneticsense elements, the method comprising: applying a first reset fieldpulse to orient a plurality of magnetic sense elements in a first groupof a first bridge circuit, the first bridge circuit configured to sensean external magnetic field along a first sense axis; applying a firststabilization field to the plurality of magnetic sense elements in thefirst group of the first bridge circuit; determining a component of anexternal field in a first direction as sensed by the plurality ofmagnetic sense elements in the first group of the first bridge circuit,wherein the first direction is a reference layer magnetization directionof a second bridge circuit that is configured to sense an externalmagnetic field along a second sense axis that is orthogonal to the firstsense axis; determining, based on the determined component of theexternal field, a magnitude of a second reset field pulse and of asecond stabilization field as compared to a default stabilization field;applying at least one of (i) the second reset field pulse to a pluralityof magnetic sense elements in a second group of the second bridgecircuit prior to measuring a direction and magnitude of an externalfield, and (ii) the second stabilization field while measuring adirection and magnitude of an external field utilizing the plurality ofmagnetic sense elements of the second group of the second bridgecircuit; and measuring a direction and magnitude of an external fieldutilizing the plurality of magnetic sense elements of the second groupof the second bridge circuit.
 2. The method of claim 1, wherein applyingat least one of (i) the second reset field pulse to a plurality ofmagnetic sense elements in a second group of the second bridge circuitprior to measuring the direction and magnitude of the external field,and (ii) the second stabilization field while measuring the directionand magnitude of the external field utilizing the plurality of magneticsense elements of the second group of the second bridge circuitcomprises: applying the second reset field pulse in a direction opposedto a direction of the default stabilization field if the determinedcomponent of the external field is in an opposite direction of thedefault stabilization field and greater than at least one of 175% of thedefault stabilization field and (ii) half a sense element anisotropy;and applying the second stabilization field that is approximately thesame as the default stabilization field.
 3. The method of claim 1,wherein applying the second reset field pulse to a plurality of magneticsense elements in a second group of the second bridge circuit prior tomeasuring the direction and magnitude of the external field comprises:applying the second reset field pulse in a direction opposed to adirection of the default stabilization field if the determined componentof the external field is in an opposite direction of the defaultstabilization field and between 75% to 175% of the default stabilizationfield.
 4. The method of claim 1, wherein applying at least one of (i)the second reset field pulse to a plurality of magnetic sense elementsin a second group of the second bridge circuit prior to measuring thedirection and magnitude of the external field, and (ii) the secondstabilization field while measuring the direction and magnitude of theexternal field utilizing the plurality of magnetic sense elements of thesecond group of the second bridge circuit comprises: applying the secondreset field pulse in a direction opposed to a direction of the defaultstabilization field if the determined component of the external field isin an opposite direction of the default stabilization field and lessthan 75% of the default stabilization field; and applying the secondstabilization field in the opposite direction of the defaultstabilization field and with a same magnitude of the defaultstabilization field.
 5. The method of claim 1, wherein applying at leastone of (i) the second reset field pulse to a plurality of magnetic senseelements in a second group of the second bridge circuit prior tomeasuring the direction and magnitude of the external field, and (ii)the second stabilization field while measuring the direction andmagnitude of the external field utilizing the plurality of magneticsense elements of the second group of the second bridge circuitcomprises: applying the second reset field pulse in a direction of thedefault stabilization field if the determined component of the externalfield is in a same direction as the default stabilization field andgreater than 175% of the default stabilization field; and applying asecond stabilization field that has a same magnitude as the defaultstabilization field and an opposite direction to the defaultstabilization field.
 6. The method of claim 1, wherein applying thesecond reset field pulse to a plurality of magnetic sense elements in asecond group of the second bridge circuit prior to measuring thedirection and magnitude of the external field comprises: applying thesecond reset field pulse in a same direction as the defaultstabilization field if the determined component of the external field isin a same direction of the default stabilization field and between 75%to 175% of the default stabilization field.
 7. The method of claim 1,wherein applying at least one of (i) the second reset field pulse to aplurality of magnetic sense elements in a second group of the secondbridge circuit prior to measuring the direction and magnitude of theexternal field, and (ii) the second stabilization field while measuringthe direction and magnitude of the external field utilizing theplurality of magnetic sense elements of the second group of the secondbridge circuit comprises: applying the second reset field pulse in asame direction as the default stabilization field if the determinedcomponent of the external field is in a same direction as the defaultstabilization field and less than 75% of the default stabilizationfield; and applying the second stabilization field that is the same asthe default stabilization field.
 8. The method of claim 1, whereinapplying at least one of (i) the second reset field pulse to a pluralityof magnetic sense elements in a second group of the second bridgecircuit prior to measuring the direction and magnitude of the externalfield and (ii) the second stabilization field while measuring thedirection and magnitude of the external field utilizing the plurality ofmagnetic sense elements of the second group of the second bridge circuitcomprises: applying the second reset field pulse, greater in magnitude,but in a same direction as a default reset field pulse if the determinedcomponent of the external field is in an opposite direction of thedefault stabilization field and less than half the sense elementanisotropy; and applying the second stabilization field that is greaterin magnitude as the default stabilization field and in a same directionas default stabilization field.
 9. The method of claim 1, whereinapplying at least one of (i) the second reset field pulse to a pluralityof magnetic sense elements in a second group of the second bridgecircuit prior to measuring the direction and magnitude of the externalfield, and (ii) the second stabilization field while measuring thedirection and magnitude of the external field utilizing the plurality ofmagnetic sense elements of the second group of the second bridge circuitcomprises: applying the second reset field pulse in a same direction asthe default stabilization field if the determined component of theexternal field is in an opposite direction of the default stabilizationfield and less than 50% of the default stabilization field; and applyingthe second stabilization field that is approximately the same as thedefault stabilization field.
 10. The method of claim 1, wherein applyingthe second reset field pulse to a plurality of magnetic sense elementsin a second group of the second bridge circuit prior to measuring thedirection and magnitude of the external field comprises: applying a hardaxis field in response to sensing an external field while applying thesecond reset field pulse.
 11. The method of claim 1, wherein applyingthe second stabilization field while measuring the direction andmagnitude of the external field utilizing the plurality of magneticsense elements of the second group of the second bridge circuitcomprises: applying a second stabilization current through a secondcurrent line adjacent the magnetic sense elements, the magnitude of thesecond stabilization current determined by the sensed external field.12. The method of claim 1, wherein applying the second stabilizationfield while measuring the direction and magnitude of the external fieldutilizing the plurality of magnetic sense elements of the second groupof the second bridge circuit comprises: adjusting a second stabilizationcurrent to set a net stabilization field to a predetermined value,wherein the net stabilization field consists of the second stabilizationfield and the external field.
 13. The method of claim 1, whereinapplying the second reset field pulse to a plurality of magnetic senseelements in a second group of the second bridge circuit prior tomeasuring the direction and magnitude of the external field comprises:adjusting the second reset field pulse for setting a net reset fieldpulse to a predetermined value, wherein the net reset field pulseconsists of the second reset field pulse and the external field.
 14. Amethod for operating a plurality of magnetic sense elements, the methodcomprising: applying a first reset current pulse to a first current lineadjacent to a plurality of magnetic sense elements in a first group, theapplication of the first reset current pulse generating a first resetfield pulse; applying a first stabilization current to the first currentline, the application of the first stabilization current generating afirst stabilization field; determining a component of an external fieldin a first direction as sensed by the plurality of magnetic senseelements in the first group; determining, based on the determinedcomponent of the external field, (i) a second reset current pulse forgenerating a second reset field pulse; and (ii) a second stabilizationcurrent for generating a second stabilization field, the second resetfield pulse and the second stabilization field for application to asecond group of sense elements; applying at least one of (i) the secondreset current pulse to the second group of sense elements prior tomeasuring a direction and magnitude of an external field, and (ii) thesecond stabilization current while measuring a direction and magnitudeof an external field utilizing the second group of magnetic senseelements; and measuring a direction and magnitude of an external fieldutilizing magnetic sense elements in a second group.
 15. The method ofclaim 14, wherein applying at least one of (i) the second reset currentpulse to the second group of sense elements prior to measuring thedirection and magnitude of the external field, and (ii) the secondstabilization current while measuring the direction and magnitude of theexternal field utilizing the second group of magnetic sense elementscomprises: applying the second reset current pulse in a directionopposed to a default stabilization current if the determined directionof the external field is in an opposite direction of the defaultstabilization field and greater than 175% of a default stabilizationfield, and applying the second stabilization current that isapproximately the same as the first stabilization current; applying thesecond reset current pulse in the direction opposed to the defaultstabilization current if the determined direction of the external fieldis in the opposite direction of the default stabilization field andbetween 75% to 175% of the default stabilization field; applying thesecond reset current pulse in the direction opposed to the defaultstabilization current if the determined component of the external fieldis in the opposite direction of the default stabilization field and lessthan 75% of the default stabilization field, and applying a secondstabilization current that is the opposite direction of the defaultstabilization field and with a same magnitude of the defaultstabilization current; applying the second reset current pulse in adirection of the default stabilization current if the determinedcomponent of the external field is in a same direction as the defaultstabilization field and greater than 175% of the default stabilizationfield, and applying the second stabilization current that hasapproximately a same magnitude as the default stabilization field and anopposite direction to the default stabilization current; applying thesecond reset current pulse in a same direction as the defaultstabilization current if the determined component of the external fieldis in the same direction of the default stabilization field and between75% to 175% of the default stabilization field; and applying the secondreset current pulse in the same direction as the default stabilizationcurrent if the determined component of the external field is in the samedirection as the default stabilization field and less than 75% of thedefault stabilization field, and applying the second stabilizationcurrent that is approximately the same as the default stabilizationcurrent.
 16. The method of claim 14, wherein applying at least one of(i) the second reset current pulse to the second group of sense elementsprior to measuring the direction and magnitude of the external field,and (ii) the second stabilization current while measuring the directionand magnitude of the external field utilizing the second group ofmagnetic sense elements comprises: applying the second reset currentpulse in a direction opposed to a default stabilization current if thedetermined direction of the external field is in an opposite directionof the default stabilization field and greater than half a sense elementanisotropy, and applying a second stabilization current that is the sameas the first stabilization current; applying the second reset currentpulse in the direction of the default stabilization current if thedetermined direction of the external field is in a same direction as thedefault stabilization field and greater than 175% of the defaultstabilization field, and applying the second stabilization current thathas a same magnitude of the default stabilization current and in anopposite direction to the default stabilization current; applying thesecond reset current pulse in the same direction as the defaultstabilization current if the determined direction of the external fieldis in the same direction of the default stabilization field and between75% to 175% of the default stabilization field; applying the secondreset current pulse in the same direction as the default stabilizationcurrent if the determined direction of the external field is in the samedirection as the default stabilization field and less than 75% of thedefault stabilization field, and applying the second stabilizationcurrent that is the opposite direction of as the default stabilizationcurrent; applying the second reset current pulse, greater in magnitudethan a default reset current and in a same direction as the defaultreset current if the determined direction of the external field is inthe opposite direction of the default stabilization field and betweenless than half the sense element anisotropy and 50% of the defaultstabilization field, and applying the second stabilization current thatis greater in magnitude than the default stabilization current and andin the same direction as default stabilization current; and applying thesecond reset current pulse in the same direction as the defaultstabilization field if the determined direction of the external field isin the opposite direction of the default stabilization field and lessthan 50% of the default stabilization field, and applying the secondstabilization current that is approximately the same as the defaultstabilization current.